In recent years, the telecommunication industry has seen the development and deployment of the first commercial nongeostationary orbit (NGSO) satellite constellations to respond to the rapidly expanding demand for global telecommunication services. Satellite communication systems incorporating such satellite constellations can be viewed as a global network of nodes having the potential to deliver a variety of information services to locations virtually anywhere in the world at a much lower cost than would be possible solely using prior art wireless and/or wire-based terrestrial networks. Among these satellite communication systems, use of satellite constellations designed with both polar and inclined orbits have been proposed.
To enable wide-spread use of such satellite-based communication systems to a variety of system users, system operators need to develop efficient, cost-effective, relatively small antennas for transmitting signals to and/or receiving signals from user terminals. Additionally, antennas for user terminals also need to provide other capabilities to enable uninterrupted communication with satellites, as discussed further below.
In NGSO satellite communication systems, satellites move at relatively rapid speeds over the surface of an underlying celestial body, such as the earth. As it sweeps over the surface of the earth, an NGSO satellite projects a satellite "footprint" made up of a number of radio frequency (RF) "beams" or "cells" toward system users located on or near the surface of the Earth. Because each NGSO satellite is visible to a particular user terminal only for relatively small increments of time (typically only for several minutes), communications having a duration of more than several minutes are handled through a "hand-off" process, which involves switching communication service between cells or beams of a single satellite footprint and between cells or beams of different satellites within the satellite communications system to ensure continuous communication.
Moreover, space-based communication systems must share a limited frequency spectrum. In the past, regulators have either exclusively licensed portions of spectrum to a system within a frequency band, or have licensed spectrum to multiple systems, designating one system as having priority to operate within a particular spectral band. One or more other systems may be required to avoid interfering with the system having such priority.
The desire to enhance the capacity of space-based communication systems has resulted in continuing evolution of antenna technology. For example, systems have been described which use phased array antennas for communicating with satellites of the system. However, prior art phased array antennas have required relatively large package sizes and are very costly.
Therefore, what is needed is a user terminal antenna system with maximum hemispherical coverage to provide satellite tracking and hand-off capability through a relatively small package available at a reasonable cost. What is further needed is a user terminal antenna which is operable in the context of a spectrum sharing scenario.